Coastal Contractor

Soundings - Swimming Pools on the Beach

: A customer wants a swimming pool
next to a beach house. We need to elevate the house above the Base
Flood Elevation, but what kind of choices do we have for siting and
constructing the pool?

A:

Ted Cushman responds: The general
goal when building a pool in an area subject to flooding from a
storm surge is the same as for any structure: the pool should be
built so as to minimize damage to itself and to nearby structures
in the event of a 100-year storm. But the specific techniques for
achieving that goal will vary depending on site conditions, as well
as on state and local rules. A registered architect or engineer
must also certify the final design.

FEMA Hurricane Ivan wiped out these beachfront pools in coastal
Alabama in 2004.

Mark Wolfe/FEMA The storm surge from Hurricane Isabel in 2003
destroyed this pool on the North Carolina Outer Banks.

Before starting a pool project, the designer and builder should
consult all applicable local, state, and federal rules, advises
Chris Jones, P.E., an engineer based in Durham, N.C., who
specializes in coastal engineering and coastal zone management.
“They need to find out whether pools are addressed by the
local building code or a local ordinance. They also have to check
for state regulations governing construction along the beachfront,
and see if there are specific requirements for pools,” says
Jones.

FEDERAL REQUIREMENTS

Jones says that federal requirements are specified by the federally
backed National Flood Insurance Program (NFIP), the only flood
insurance generally available to homeowners in flood-prone areas.
NFIP will only offer insurance where the jurisdiction has adopted
rules designed to reduce future flood risks to new
construction.

For its pool construction requirements, NFIP refers to FEMA
Technical Bulletin 5: Free-Of-Obstruction Requirements, which
provides guidance for building accessory structures in flood-prone
areas. Jones, who worked with FEMA to update Bulletin 5, says that
the document lays out general guidelines for keeping the space
between house pilings clear from any elements that might transfer
the force of wave action to the pilings. That includes attached
elements like stairs and decks, as well as unattached structures
like outbuildings, privacy walls, septic systems, and pools. For
instance, if a builder wants to put a pool or spa directly beneath
the house, the top of the pool must be made flush with the existing
grade.

Regardless of where the pool is located on the property —
whether under the house or next to it — NFIP requires a
design professional to certify that if waters do rise, it
won’t float away and damage nearby buildings. This
requirement leaves only two choices, says Jones. One is
what’s called “frangible” construction: building
the pool in the ground, assuming that it will be destroyed during a
severe storm, and designing it so that when it does fail, it breaks
up into small pieces that won’t cause much damage. The other
option is to tie the pool to a pile or column foundation. (More on
that below.)

FLORIDA REQUIREMENTS

One example of a state ordinance is Florida’s Coastal
Construction Control Line (CCCL), which is drawn on a
county-by-county basis throughout the state to protect sensitive
dunes and beaches, and typically extends just a block or two back
from the beach. Any construction activity seaward of that line
requires permission from the state.

CCCL complicates the builder’s job by adding another layer of
concern. “[FEMA’s technical bulletins] are mainly about
minimizing damage to the buildings for insurance purposes,”
says Tony McBeal, P.E., program administrator for Coastal
Construction Control Line Permitting at the Florida Department of
Environmental Protection (DEP). He says that while DEP rules
related to the Control Line do include protection for buildings,
they also seek to minimize damage to the environment.

DEP’s environmental rules have largely to do with dune
protection. Specifically, they require the builder to limit damage
to dunes during construction. The result may be that only minimal
excavation is allowed, effectively forcing the builder to elevate
the pool above grade level.

Where in-ground pools are allowed, McBeal says that frangible
construction has fallen out of favor with builders, designers, and
inspectors. Instead, the pool will probably need to be reinforced
and pinned in place. How that is done depends on the engineer who
certifies the project, but a common solution is to build a
6-inch-thick reinforced-concrete pool shell, tied to a deep
concrete or treated-wood pile foundation. “If a storm exposes
the pool and tries to move it, the pool will not float into the
house,” he explains.

He says that all the concrete elements of this assembly must be
tied together with reinforcing steel, and the pool is made thickest
at the bottom, where it connects to the piles. That way, if waves
scour the supporting soil away from the bottom of the shell and
begin to pound on the sides or bottom, the piles won’t punch
through the bottom of the shell. The pool shell may fracture, but
the anchoring piles won’t allow it to float or drift as a
unit toward the house, where waves could pound it against the
home’s pilings.

ELEVATED POOLS

The design of an elevated pool will depend in part on how high it
is elevated. If the builder doesn’t want to raise the house
higher than the minimal required elevation, but wants the pool deck
flush with the home’s first floor, then the base of the pool
will still be in the zone of wave action. McBeal’s agency has
approved that solution in some cases, but getting the approval
required some heavy-duty engineering. “We required the bottom
of the pool to be no lower than the FEMA Base Flood
Elevation,” he says. “But because the pool would still
be impacted by waves, it had to be designed to withstand wave loads
at this elevation.”

He says that in most of these cases, the engineer’s solution
was to design a structural pool shell, like a big coffer dam
— heavily reinforced, and anywhere from 8 inches to 12 inches
thick. A regular 6-inch pool shell was then dropped inside of the
structural shell. The combined thickness sometimes totaled almost
20 inches.

Because these elevated pools would see stress from wave action, the
DEP also required them to be structurally isolated from the house.
“In most cases that meant leaving a gap or joint —
again, specified by the certifying engineer — between the
pool and the house, so that the stress caused by waves hitting the
pool shell would not be transmitted to the house,” says
McBeal.

Of course the designer could choose to elevate both structures
— house and pool — high enough to raise them above the
zone of wave action. That would avoid some of the engineering
involved in making the pool strong enough to resist waves, and the
two structures might not have to be made independent.

Approval of that design, like nearly everything else, will likely
depend on the local building official. In Florida, for instance,
when the state adopted a new statewide building code in 2002, it
handed local building departments the task of reviewing and
permitting pools, even seaward of the Control Line. Some local
Florida officials say that DEP standards are still observed at the
local level, but that’s not guaranteed. “You really
have to ask local building officials how they are implementing the
code,” says McBeal.

Bottom line, says McBeal: “You have to satisfy all the
federal, state, and local criteria. And the most stringent of those
will normally dictate the design of the structure.”